(a) Field of the invention:
The present invention relates to a range-finding optical system and, more particularly, to a range-finding optical system arranged to compensate the displacement of the focus point which occurs in a photographic lens system because of temperature variation.
(b) Description of the prior art:
In recent years, lenses made by molding of synthetic resin (hereinafter referred to as plastic lenses) are being widely used for photographic lens systems to be used with still cameras, video cameras, etc. This is because they have various merits which cannot be obtained from conventional glass lenses, for example, a lower cost of production, higher degre of freedom at the time of forming of aspherical surfaces, and so forth. However, as synthetic resin materials (i.e., plastic materials) generally have temperature coefficients of refractive index and coefficients of thermal linear expansion which are several ten times larger compared with optical glass materials, a large displacement of the focus point occurs when a plastic lens is used in a photographic lens system. Besides, when a camera body and lens holder are formed by using plastic materials, a displacement of the focus point is caused also by expansion and contraction of those members to be caused by temperature variation.
For example, FIG. 11 shows an example of a photographic lens system to which the present invention is to be applied. In said figure, reference symbols L.sub.1, L.sub.2 and L.sub.4 respectively represent glass lenses, reference symbol L.sub.3 represents a plastic lens, and reference symbol S represents an aperture stop. In case of the above-mentioned photographic lens system, when the temperature decreases, the plastic lens L.sub.3 causes such variation that the refractive index thereof becomes higher and, moreover, radii of curvature of the surfaces thereof become smaller. Chiefly as a result of the above-mentioned variation, the image point is displaced by .DELTA.a toward the lens system compared with the case of normal temperature.
FIG. 12 shows the amount of displacement .DELTA.a of the focus point which occurs in the temperature range of 50.degree. C. to -10.degree. C. when the photographic lens system shown in FIG. 11 is adjusted so that said lens system can be properly focused when the temperature is 20.degree. C. As shown in said figure, the displacement .DELTA.a of the focus point varies approximately linearly in relation to the temperature variation.
FIGS. 13 and 14 respectively show graphs illustrating the relation between the object distance and contrast of the image of the object in case of a camera arranged to perform automatic focusing of the photographic lens system thereof based on a detected signal from a focal point detecting system which is provided separately from the photographic lens system. For said photographic lens system, the zone focusing method is adopted, i.e., the photographic lens system is driven so that it stops at positions where the lens system is correctly focused on a particular object at the object distances of 1.3 m, 2 m and 4 m respectively and, at respective stopping positions, the photographic lens system covers the zones of object distances respectively from the shortest object distance up to P.sub.1, from P.sub.1 up to P.sub.2 and from P.sub.2 up to the infinite object distance by dividing the whole range of object distance into those three zones. In FIGS. 13 and 14, the axis of abscissa is graduated that the reciprocals of respective object distances come to positions at regular intervals.
As it is evident from FIG. 13, at the normal temperature (20.degree. C., the contrast of the image of the object becomes the most favourable at the object distances of 1.3 m 2 m and 4 m respectively and, at each of the boundary points P.sub.1 and P.sub.2, the contrast becomes equal to that of the adjacent zone. However, when the temperature decreases to the low temperature (-10.degree. C.), the focus point of the photographic lens system is displaced (by -0.07 mm) as shown in FIG. 12 and, consequently, the object distance at which the most favourable contrast is obtained on the surface corresponding to the film surface somewhat shifts toward the short object distance side in each of said zones. As a result, a range where the contrast of the image of the object becomes considerably unfavourable appears on the long object distance side in each zone and, therefore, it is impossible to obtain a photograph with a sharp image.
To solve the above-mentioned problem, Japanese published unexamined patent application No. 64204/82 proposes a means arranged that a temperature detecting element is provided to a camera and, by compensating the distance data based on the temperature data from said detecting element, the positions of the boundary points P.sub.1 and P.sub.2 between respective zones are somewhat shifted, in other words, the stopping positions of the photographic lens system is changed over at positions of object distances somewhat shorter than those of the normal temperature. Besides, Japanese published unexamined patent application No. 160129/84 proposes a method to shift the change-over points of the positions of the photographic lens system by varying the optical alignment of the range-finding device by using a bimetal member.
However, the above-mentioned known means and method have disadvantages that it is necessary to additionally provide a new and particular member exclusively for the purpose of compensation of the focus point to be caused by temperature variation and, therefore, the cost of production becomes high.